I have a couple of 12bh7a tubes. They work fine as phase splitters (quite) but are noisy (hum) in the center position. 12au7 is quiet in center spot. 2 bad tubes? Wiring error? Any ideas appreciated.
5 posters
VTA ST-70 and 12bh7
fzapped- Posts : 7
Join date : 2018-05-19
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
fzapped wrote:I have a couple of 12bh7a tubes. They work fine as phase splitters (quite) but are noisy (hum) in the center position. 12au7 is quiet in center spot. 2 bad tubes? Wiring error? Any ideas appreciated.
The center driver tube is the voltage amplifier for BOTH channels. The two side tubes are the phase splitter/inverter tubes for that channel alone. Because their of different function in the amp, the two side tubes are relatively immune to causing any hum/noise in the amp.
Bob
fzapped- Posts : 7
Join date : 2018-05-19
Noisy tubes then?
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Kentley- Posts : 496
Join date : 2015-03-06
Age : 71
Location : Worcester, MA
Yes, fz. I've often used fine old 12BH7s in that center position (my faves are vintage TungSols and Sylvanias) and they are completely silent.
Tubes4ever- Posts : 167
Join date : 2015-07-14
Location : Star, Idaho
fzapped wrote:Noisy tubes then?
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Yes, Noisy tubes. Both of your 12bh7s have 60hz "noise" leaking from the heater to the cathodes.
fzapped- Posts : 7
Join date : 2018-05-19
Since they work as phase splitters my money is not totally wasted.
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fzapped- Posts : 7
Join date : 2018-05-19
Ok a year has gone by and I just bought a new Electro harminix 12BH7A Gold Pin and I got hum. Put back 12au7 and no hum. Am I just unlucky?
peterh- Posts : 1833
Join date : 2012-12-25
Location : gothenburg, sweden
No. This happens, not frequent, but happens. That's why you should keep some spares at home.fzapped wrote:Ok a year has gone by and I just bought a new Electro harminix 12BH7A Gold Pin and I got hum. Put back 12au7 and no hum. Am I just unlucky?
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